Memory control device and semiconductor processing apparatus

ABSTRACT

The present invention provides a memory control device and a semiconductor processing apparatus which can be flexibly made adapted to a plurality of kinds of semiconductor memories. An SDRAM controller has: a register unit to which a command to be issued and a minimum interval (wait time) between issue of the command and issue of the next command are written by a CPU; and a command issuing unit that stops issue of the next command until the minimum interval written in the register elapses since issue of the command written in the register. Therefore, by changing software for the CPU, the SDRAM controller can be flexibly adapted to a plurality of kinds of SDRAMs.

CROSS-REFERENCE TO RELATED APPLICATIONS

The disclosure of Japanese Patent Application No. 2007-251591 filed onSep. 27, 2007 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a memory control device and asemiconductor processing apparatus and, more particularly, relates to amemory control device and a semiconductor processing apparatus forcontrolling a semiconductor memory by using a series of commands.

Hitherto, an image processing apparatus for compressing/decompressing animage is provided with an SDRAM (Synchronous Dynamic Random AccessMemory) for storing a large amount of data, and an SDRAM controller forcontrolling the SDRAM (refer to, for example, Japanese Unexamined PatentPublication No. 2000-10856).

In the control of the SDRAM, for initialization at the time of power-on,a change in the drive frequency, return to a normal mode from alow-power mode, calibration, or the like, a command issuing sequence forissuing a series of SDRAM commands at sufficient intervals has to beexecute. As a limitation of the command intervals, usually, only theminimum value is specified. Concrete length of the minimum intervalvaries from a few cycles to hundreds of microseconds (tens of thousandsof cycles).

As a first method of executing the command issuing sequence, the SDRAMcontroller is provided with a controller as dedicated hardware (statemachine) and the sequence is automatically executed by the controller.

As a second method of executing the command issuing sequence, aninstruction to issue an SDRAM command is given from an externalcontroller to the SDRAM controller to make the SDRAM controller issue anSDRAM command. In the second method, by repeatedly giving the commandissue instruction from the external controller, the command issuingsequence is executed.

SUMMARY OF THE INVENTION

The first method has, however, a disadvantage from the viewpoint ofadaptation to a plurality of kinds of SDRAMs. The command issuingsequence varies according to the kinds of SDRAMs and also slightlyvaries among manufacturers.

The sequence of calibration or the like is complicated and thestrategies to be employed also vary according to a system to which thesequence is assembled. The controller realized by fixed hardware is notflexible.

In the second method, a method of accurately setting the intervals ofSDRAM commands is an issue. When the interval from an SDRAM command tothe next SDRAM command is measured by an external controller, thefollowing problems occur.

There are many factors to be considered in order to assure a sufficientcommand interval, and the function is spread to a number of modules.Consequently, the specifications of the external controller arecomplicated, and it is also difficult to perform verification. Softwareis excessively complicated, and it causes deterioration in readabilityand maintenance. The process load on the external controller alsoincreases. Since the unpredictable part is large, when a sufficientmargin is assured, the device waits more than necessary.

A main object of the present invention is therefore to provide a memorycontrol device and a semiconductor processing apparatus which can beflexibly adapted to a plurality of kinds of semiconductor memories.

The present invention provides a memory control device for controlling asemiconductor memory by issuing a series of commands and including aregister and a command issuing unit. To the register, a command to beissued and wait time for issue of the next command since the command isissued are written by an external control device. The command issuingunit stops issuing the next command until the wait time written in theregister elapses after the command written in the register is issued.

A semiconductor processing apparatus of the invention includes a systembus, a CPU, and a memory control device. The CPU is coupled to thesystem bus. The memory control device is coupled to the system bus andreceives command information from the CPU and time interval informationthat specifies a time interval as a wait time of the command informationuntil the next command information is issued to an externalsemiconductor memory. The memory control device includes a register forstoring the command information and the time interval information. Theregister receives the command and the time interval which are output tothe system bus, and stores them in a coupled state.

In the memory control device and the semiconductor processing apparatusaccording to the present invention, when a command to be issued and waittime for issue of the next command since the command is issued arewritten in the register by an external control device such as a CPU,issue of the next command is stopped until the wait time written in theregister elapses after the command written in the register is issued.Therefore, by changing software of the external control unit, the memorycontrol device and the semiconductor processing apparatus can be madeflexibly adapted to a plurality of kinds of semiconductor memories.Since the interval of commands is controlled by the memory controldevice, the load on the external control device can be lessened.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing main components of a memory system asan embodiment of the present invention.

FIG. 2 is a block diagram for explaining signals transmitted/receivedbetween an SDRAM controller and an SDRAM shown in FIG. 1.

FIG. 3 is a block diagram showing a schematic configuration of the SDRAMcontroller illustrated in FIG. 1.

FIG. 4 is a time chart showing operations of the memory systemillustrated in FIGS. 1 to 3.

FIG. 5 is another time chart showing operations of the memory systemillustrated in FIGS. 1 to 3.

FIG. 6 is a block diagram showing a concrete configuration of an SDRAMcontroller illustrated in FIG. 3.

FIG. 7 is a time chart showing operations of the SDRAM controllerillustrated in FIG. 6.

FIG. 8 is a block diagram showing a modification of the embodiment.

FIG. 9 is a block diagram showing another modification of theembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram showing main components of a memory system asan embodiment of the present invention. In FIG. 1, the memory system hasa semiconductor processing apparatus 4 including a CPU (CentralProcessing Unit) 1, a system bus 2, and an SDRAM controller 3, and anSDRAM 5. The “SDRAM” is a generic term of various SDRAMS such as SDR(Single Data Rate), DDR (Double Data Rate), LP (Low Power)-DDR, DDR2,and DDR3.

Signals between the SDRAM controller 3 and the SDRAM 5 can be classifiedto clocks, commands, data, and the others as shown in FIG. 2. The clockis a signal as a reference of the operation timing of the SDRAM 5. Thecommand signals include a clock enable signal CKE, a chip select signalCS#, a row address/strobe signal RAS#, a column address/strobe signalCAS#, a write enable signal WE#, an address signal A, and a bank addresssignal BA. The data signals include a data signal DQ, a signal DQSindicative of an output timing of the data signal DQ, and a data masksignal DM. The other signals include a reset signal RESET# forinitializing a control circuit at power-on, a signal ODT for turningon/off a terminating resistor of a data output line, and the like.

The SDRAM command is an instruction to the SDRAM 5, expressed by acombination of command signals input to the SDRAM synchronously withclocks. As also described in the background art, at power-on or at thetime of return from the low-power mode, the SDRAM controller 3 has tooutput a series of SDRAM commands to the SDRAM 5 at sufficient timeintervals. As the limitation of the command intervals, usually, only theminimum value is specified. Concrete length of the minimum intervalvaries from a few cycles to hundreds of microseconds (tens of thousandsof cycles).

The kinds of the SDRAM commands necessary for the command issuingsequence of the present invention include mode register set, prechargeall banks, refresh, ZQ calibration, CKE operation, and the like. The CKEoperation includes power down entry, power down exit, self-refreshentry, self-refresh exit, and the like. In addition, an operation of aRESET# terminal is also necessary in some cases.

FIG. 3 is a diagram showing the configuration of the principle of theSDRAM controller 3. In FIG. 3, the SDRAM controller 3 includes aregister unit 6 and a command issuing unit 9. The register unit 6includes a first storage 7 for storing a first signal indicative of anSDRAM command to be issued, and a second storage 8 for storing a secondsignal indicative of the minimum interval in which the register unit 6is to wait since the SDRAM command indicated by the first signal writtenin the first storage 7 is issued until the next SDRAM command is issued.Each of the first and second signals includes a data signal made of aplurality of bits. The storages 7 and 8 may be formed by storage devicesas hardware, or software storage areas assigned to specified addressesto/from which data is written/read by the CPU 1.

According to a program stored in a not-shown memory, the CPU 1sequentially writes a plurality of sets of first and second signals tothe register unit 6 to make the SDRAM controller 3 issue the series ofSDRAM commands. In the case where the bit widths of the register unit 6and the system bus 2 are the same, the CPU 1 writes the set of thecommand and the “minimum interval” in a coupled state into the registerunit 6. For example, when the bit width of the register unit 6 is 64bits and the bit width of the system bus 2 is 32 bits, the CPU 1 writesdata of 32 bits twice into the register unit 6 so as to maintain thecoupled state. In the register unit 6, the command and the “minimuminterval” is held in a state where the command and the “minimuminterval” are coupled.

Since the storage capacity of the register unit 6 is limited, when thestorages 7 and 8 do not have a free space, the register unit 6 side hasto make CPU 1 wait. Consequently, the CPU 1 sends a write request to theregister unit 6. When the storages 7 and 8 have free space, the registerunit 6 receives the write request of the CPU 1 and writes the first andsecond signals to the storages 7 and 8, respectively. In the case wherethe storages 7 and 8 do not have free space, the register unit 6 doesnot receive the write request of the CPU 1 until space becomesavailable, and the CPU 1 waits.

The command issuing unit 9 issues the SDRAM command indicated by thefirst signal written in the first storage 7 and waits only for the timeof the minimum interval indicated by the second signal written in thesecond storage 8, since the SDRAM command is issued until the next SDRAMcommand is issued. During the period in which the command issuing unit 9waits, the command issuing unit 9 does not output valid SDRAM commands.Invalid SDRAM commands include a no operation command NOP, and a devicedeselect command DES for fixing the chip select signal CS# at aninactivate level.

In other words, at the time of processing a command issue requestentered by the n-th writing to the register unit 6, the command issuingunit 9 makes a check to see whether the “minimum interval” designated bythe (n−1)th writing has elapsed since the SDRAM command issue by then−1th writing to the register unit 6. The command issuing unit 9 waitsuntil the designated interval is assured, and issues a commanddesignated by the n-th writing.

The SDRAM 5 receives the SDRAM command issued by the command issuingunit 9 and performs an operation according to the received SDRAMcommand.

FIGS. 4A and 4B are a time chart showing the command issuing sequence ofthe memory system. It is assumed that C1 is written as a command kindand T1 is written as the minimum interval by the first writing to theregister unit 6. The command issuing unit 9 outputs the command kind C1written in the register unit 6 to the SDRAM 5. Although there is thepossibility that it takes time of a few cycles since the writing to theregister 6 till output to the SDRAM 5, it is not a big issue.

It is assumed that, before time T1 elapses since the first commandoutput, the second writing (a command kind C2 and a minimum interval T2)is performed to the register unit 6. In this case, until the interval T1since the first command issue is assured, the second command issue isdelayed.

It is assumed that the third writing (a command kind C3 and a minimuminterval T3) to the register unit 6 is performed. In the case where thethird writing timing is later than time after lapse of time T2 since thesecond command output as shown in FIGS. 4A and 4B, the designatedcommand C3 is immediately output. By performing such a control, at leastan interval equal to or longer than the minimum interval which isdesignated can be assured.

As shown in FIGS. 5A and 5B, as a kind of the command, an NOP commandcan be also selected. By selecting the NOP command, wait time sincepower-on or start of supply of clocks until an effective command isoutput can be assured. In the case where the minimum interval desired tobe designated is too long to be expressed by bit widths prepared in theregister unit 6, the intervals can be added. For example, it is assumedthat wait time of 500,000 cycles is desired to be assured and the valuewhich can be designated as the minimum interval in the register unit 6is up to 60,000 cycles. In this case, when writing of the command kindNOP and the minimum interval of 50,000 cycles to the register unit 6 isperformed ten times, desired time can be assured.

FIG. 6 is a block diagram showing a concrete configuration of the SDRAMcontroller 3. In FIG. 6, the SDRAM controller 3 has a bus interface 11,an internal bus 12, an internal buffer 13, a command issuing register14, a down counter 15, a control unit 16, a multiplexer 17, a completiondetermining unit 18, and an operation completion wait register 19. Thebus interface 11, the internal bus 12, the internal buffer 13, thecommand issuing register 14, the completion determining unit 18, and theoperation completion wait register 19 configure the register unit 6. Thedown counter 15, the control unit 16, and the multiplexer 17 configurethe command issuing unit 9.

The CPU 1 sends a write request to the SDRAM controller 3 via the systembus 2. The write request is given to the internal buffer 13 via the businterface 11 and the internal bus 12. The internal buffer 13 receivesthe write request in the case where there is room to store a commandissuing request (the SDRAM command and the minimum interval). In thecase where there is no room, the internal buffer 13 does not receive thewrite request until room is created.

When the write request is received, an SDRAM command to be issued andthe minimum interval (standby time) since the SDRAM command is issueduntil the next SDRAM command is issued are written from the CPU 1 to theinternal buffer 13 via the system bus 2, the bus interface 11, and theinternal bus 12.

The internal buffer 13 is provided to reduce time loss caused byhandshake or the like of the bus protocol and buffers a plurality ofsets of command issue requests (each made of the SDRAM command and theminimum interval). When there is room in the command issuing register14, the command issue request held in the internal buffer 13 istransferred to the command issuing register 14. The command issuingregister 14 holds the command issue request waiting for an issuingtiming. The command issuing register 14 may be a register of a one-stageconfiguration or a register having a multi-state configuration such asan FIFO. The case of a register of a one-stage configuration will bedescribed below. The “minimum interval” and “SDRAM command” held in thecommand issuing register 14 are supplied to the down counter 15 and themultiplexer 17, respectively.

The down counter 15 holds remaining time until the interval designatedby the command issue of last time is finished. The count value isdecremented every cycle. When the remaining time becomes zero, the countvalue is not decremented. When the command issue request is supplied tothe command issuing register 14 and the remaining time output from thedown counter 15 is zero (or sufficiently small), the control unit 16outputs a pulse signal indicative of a command issue timing.

In response to the pulse signal, the down counter 15 loads the value ofthe “minimum interval” held in the command issuing register 14 andstarts down-counting in preparation for issuing the next command. Themultiplexer 17 receives the SDRAM command from the command issuingregister 14, outputs the SDRAM command from the command issuing register14 for only one cycle in response to a pulse signal, and outputs aninvalid SDRAM command (for example, NOP command) in the other cycles. Inresponse to the pulse signal, data stored in the command issuingregister 14 is erased so that a free space is created.

The completion determining unit 18 determines whether all of commandissue requests loaded to the SDRAM controller 3 have been handled andoutput to the SDRAM 5 or not. After lapse of predetermined time (timecorresponding to latency from the command issuing register 14 to theSDRAM) after both of the internal buffer 13 and the command issuingregister 14 becomes empty, an output signal of the completiondetermining unit 18 becomes true (for example, the “H” level). When anew command issue request is loaded to the internal buffer 13 or thecommand issuing register 14, an output signal of the completiondetermining unit 18 becomes false (for example, the “L” level). To aread request from the CPU 1, when the output signal of the completiondetermining circuit 18 is true, the operation completion wait register19 immediately sends a reply. However, while the output signal is false,the operation completion wait register 19 does not send a reply.Consequently, the interval between a timing which is not controlled bythe SDRAM controller 3 (such as interruption of a clock or power) and anSDRAM command issue can be also assured accurately.

In place of the operation completion wait register 19, a status registerfor immediately sending to a signal indicating whether the output signalof the completion determining circuit 18 is true or false in response tothe read request of the CPU 1 may be provided. It is also possible toeliminate the operation completion wait register 19 and supply an outputsignal of the completion determining unit 18 directly to the CPU 1.

FIGS. 7A to 7G are a time chart showing a command issue sequence of thememory system illustrated in FIG. 6. At time t1, a write request is sentfrom the internal buffer 13 to the command issuing register 14. Sincethe command issuing register 14 has free space, the SDRAM command C1 tobe issued and the minimum interval T1=6 are written from the internalbuffer 13 to the command issuing register 14. Since the count value ofthe down counter 15 is zero, a pulse signal indicative of an issuetiming is output, and the SDRAM command C1 is output to the SDRAM 5. Inresponse to the pulse signal indicative of the issue timing, the minimuminterval T1=6 is loaded to the down counter 15. The down counter 15starts down-counting. Data in the command issue register 14 is erased,and the register 14 becomes empty. A busy signal indicating that theregister 14 is not empty becomes the “L” level which is an inactivelevel.

At time t2 before lapse of the minimum interval T1=6 after the firstcommand output, the write request is sent from the internal buffer 13 tothe command issue register 14, and the second writing (the command kindC2 and the minimum interval T2=4) to the register 14 is performed. Inthis case, the command issue of the second time is delayed until theminimum interval T1=6 since the command issue of the first time isassured.

Further, at time t3 after lapse of the minimum interval T2=4 after thecommand output of the second time, a write request is sent from theinternal buffer 13 to the command issue register 14, and writing (thecommand kind C3 and the minimum interval T3=10) of the third time isperformed. In this case, the minimum interval T2 has already lapsedsince the command output of the second time, so that the designatedcommand C3 is immediately output. By performing such a control, at leastthe designated minimum interval can be assured.

In the embodiment, the CPU 1 sequentially designates a series of SDRAMcommands in accordance with a program (software). Hardware (the SDRAMcontroller 3) counts the intervals of the SDRAM commands. Consequently,without complicating the specifications of the hardware so much, thedevice can be made adapted to the SDRAMs 5 of various kinds and controlmethods by changing software. In this case, simple software forcontinuously writing data to the register unit 6 is sufficient. It isunnecessary to measure time on the software side by a timer and a waitloop. Therefore, the invention is particularly effective in an LSIdesired to be compatible with number of kinds of SDRAMs. If this is notthe case, there are effects that reusability of hardware improves andthe number of designing and verifying processes is decreased.

FIG. 8 is a block diagram showing a modification of the embodiment andis compared with FIG. 3. Referring to FIG. 8, in the modification, theminimum interval between commands is written in the register unit 6 by afloating point method. In the floating point method, the minimuminterval T is expressed by a mathematical equation of T=B×2^(KA) where Kdenotes a constant which is, for example, 4, “A” denotes an exponentpart, and B denotes a mantissa part. The second storage 8 in theregister unit 6 includes a first storage area 8 a in which the exponentpart “A” of the minimum interval T is written, and a second storage area8 b in which the mantissa part B of the minimum interval T is written.In the modification, the wait time T in a wide range can be set with thesmall bit width.

FIG. 9 is a block diagram showing another modification of theembodiment. In the modification of FIG. 9, the internal buffer 13 inFIG. 6 is replaced with an internal buffer 20. Depending on the kinds ofSDRAMs (mode register set, refresh, ZQ calibration, or the like), theSDRAM 5 does not accept a new SDRAM command for a predetermined periodafter an SDRAM command is issued. In the modification, in the case wherea value designated in an “interval” field is a special value (forexample, all bits “0”) by writing from the CPU 1, the value is replacedwith a default value and the default value is written in the commandissuing register 14. The default value is selected according to the kindof the command designated in the writing.

Specifically, the internal buffer 20 includes a bus protocol controlunit 21, a format converter 22, and a buffer 23. In the case where thereis a write request from the CPU 1, when the buffer 23 has free space,the bus protocol control unit 21 receives the write request. When thebuffer 23 does not have free space, the bus protocol control unit 21does not accept the write request until a free space is created. Whenthe write request is accepted, the command and the minimum interval arewritten in the format converter 22.

The format converter 22 checks the written command and minimum interval.When the minimum interval is the special value, the default valuedetermined according to the kind of the command is output as the minimuminterval to the buffer 23. The default value is given from the outsideof the internal buffer 20 (for example, the CPU 1) in order to increaseflexibility and re-usability of the circuit. The format converter 22outputs the command as it is to the buffer 23. As shown in FIG. 6, theminimum interval held in the buffer 23 is given to the down counter 15,and the command is given to the multiplexer 17. In the modification, theflexibility and re-usability of the circuit can be further increased.The command is output via the format converter 22. It is intended tooutput the command and the minimum interval to the buffer 23 at the sameinput timing. For example, the timings can be adjusted by a simplecircuit such as a flip flop.

It should be understood that the embodiments disclosed are illustrativeand not restrictive in all respects. The scope of the invention isdefined by the claims rather than by the above description and allchanges that fall within meets and bounds of the claims or equivalenceof such meets and bounds are intended to be within the scope of theinvention.

What is claimed is:
 1. A memory control device for controlling asemiconductor memory by issuing a series of commands, comprising: aregister to which a command to be issued and wait time for issue of thenext command since the command to be issued is issued are written by anexternal control device; a command issuing unit that stops issuing thenext command until the wait time written in the register elapses afterthe command written in the register is issued, and a determining unitfor determining whether issue of a command written in the register hasbeen completed or not, wherein en an inquiry of whether issue of thecommand is completed or not is received from the external controldevice, the determining replies whether issue of the command has beencompleted or not.
 2. The memory control device according to claim 1,wherein the command issuing unit also issues an NOP command forinstructing waiting without performing anything on the semiconductormemory.
 3. The memory control device according to claim 1, wherein thewait time is expressed by a floating point method.
 4. The memory controldevice according to claim 1 , wherein when a predetermined value iswritten as the wait time, the command issuing unit stops issuing thenext command until time determined by the command written in theregister elapses.
 5. The memory control device according to claim 1,wherein a request to access a specific address received from theexternal control device is not responded until issue of the command iscompleted.